1. Field of the Invention
This invention relates to hydraulic working oil compositions for use in buffers and more particularly to such oil compositions suitable for car suspension devices such as shock absorbers, active suspensions, stay dampers and engine dampers.
2. Prior Art
As conventional hydraulic working oils which have hitherto been used in car buffer devices such as shock absorbers, active suspensions, stay dampers and engine dampers, there have been known those incorporated with a phosphoric acid ester and/or a phosphorus acid ester to provide them with friction-reducing properties and wear-preventing properties. In addition, there have also been widely used such hydraulic working oils in which a fatty acid, aliphatic alcohol, fatty acid ester, aliphatic amine and other oily agents are used to further improve the working oils in friction-reducing properties.
For example, a fluid composition for use in shock absorber comprising a boron-containing dispersant and phosphorus-containing compounds such as dihydrocarbyl phosphoric acid ester, monohydrocarbyl phosphoric acid ester and dihydrocarbyl phosphoric acid ester in a lubricating base oil is disclosed in U.S. Pat. No. 4,634,543. In addition, a power transmission fluid comprising as an essential component a phosphorous acid triester selected from triaryl phosphite or trialkylaryl phosphite and hydroxylamine in lubricating oil is disclosed in U.S. Pat. No. 5,078,893.
Hydraulic working oils are those which are required to be capable of reducing friction at friction surfaces simultaneously with preventing wear of the friction surfaces. Recently, there have been increasingly used bush members impregnated with a Teflon resin in attempts to reduce friction at friction surfaces from the standpoint of material or substance used. Further, particularly gas-sealed type and damping force-variable type buffer have increasingly been used and, therefore, load applied to the friction surfaces of buffers has been increased and conditions under which the buffers are used have come to be severe.
With a change in such conditions or circumstances under which hydraulic working oils are additionally used, conventional such oils incorporated only with a phosphoric acid ester and/or a phosphorous acid ester, or those in which a fatty acid, aliphatic alcohol, a fatty acid ester and other oily agents are used, have raised problems that they will allow friction surfaces to be considerably worn with their friction-reducing effects being low and poor. In addition, it has been found that the conventional working oils in which is used a phophoric acid ester and/or a phosphorous acid ester as well as an aliphatic amine which is an oily agent, will exhibit somewhat excellent wear-preventing and friction-reducing effects on the friction surfaces of a suspension device at the initial time when the working oils are used therein but the conventional working oils will raise problems that they cannot keep such effects durably and will greatly increase in frictional coefficient during their use whereupon spherical particles produced by the wear of the friction surfaces are attached to the friction surfaces which are the surfaces of a Teflon resin-impregnated bush member.
There has been a great desire to develop novel hydraulic working oils for a buffer which meet new requirements such as excellent applicability to Teflon resin-impregnated bush members and excellent durability (little degradation during the use) of friction-reducing and wear-preventing effects, in addition to the conventional requirements, since the new requirements have become significant.